Mac OS X only: System utility RightZoom runs in the background and modifies the OS X maximise behaviour to fill the whole screen—perfect for readers that recently made the switch to Mac.
The default OS X maximise behaviour increases the size of the window to fit the contents instead of the Windows or Linux maximise, which fills the entire screen. RightZoom changes that behavior—just launch the application and your windows will start maximising to fill the whole screen just like you might expect. You can configure the settings to exclude a list of applications, or you can tell the utility to only work for certain applications—either way, it's a great tool for recent Mac converts who haven't or can't get used to the difference.
RightZoom is a free download for Mac OS X only. Hit the link for the download, or check out the Switching to Mac walk-through for a screenshot tour.